This invention relates to digital communication systems and, more particularly, to apparatus for mitigating digital signal degradation during system overload.
In the digital communication system disclosed in U.S. Pat. No. 4,002,841, issued Jan. 11, 1977 to Y. C. Ching et al. and entitled "Data Compression Using Nearly Instantaneous Companding in a Digital Speech Interpolation System", inactivity time is utilized to reduce the bit rate on a link between transmitter and receiver by compressing digital characters from a plurality of trunks onto a lesser plurality of channels. If the number of trunks having, for example, active speech or data samples, as detected by a speech or data detector, exceeds the number of channels, an overload may exist. During overload, it is common to truncate a bit from a transmitted character. By so reducing the number of bits employed to encode the sample, the effective number of channels on the link can be increased. However, bit truncation leads to signal degradation. The prior art mitigates some deleterious effects of overload by a priority trunk rotation arrangement. In particular, the order of assigning, for example, active speech samples from trunks to channels is rotated among all trunks, thereby achieving a structured control over the signal degradation as bits are truncated during overload. Unfortunately, the degradation to signals from one trunk may not be uniform when compared with the degradation to signals from a second trunk. To illustrate this nonuniformity, we offer the following partial review of the prior art.
Digital characters can be multiplexed into a time slot of a 193-bit frame and the frame can be provided to one of a plurality of, for example to one of eight, input terminals of a digital transmitter. The frame, usually provided at a nominal bit rate of 1.544 Mb/s, typically includes a one-bit framing signal and a plurality of eight-bit characters, each character representing, for example, a speech sample from a different one of a plurality of different trunks, typically 24 trunks. For ease of description, it is assumed that an input frame is provided concurrently to each input terminal of the transmitter. Hence, the system is assumed able to process characters from the (24 .times. 8 =) 192 trunks in about 125 microseconds, that time being related to the reciprocal of the input bit rate. The processing includes the development of an output multiframe to be provided to a transmitter output terminal for transmission to the receiver at a nominal bit rate of about 3.152 Mb/s. One output multiframe includes 24 output frames, each output frame having a format somewhat different than the format of an input frame. Specifically, a 394-bit output frame includes a four-bit framing signal, a 24-bit status field, and a 366-bit data field. Hence, the time to transmit an output frame is also about (394 bits .div. 3.152 Mb/s =) 125 microseconds.
The status field of an output frame is for signalling the activity status for each of eight trunks, while the data field is for interleaving bits of digital characters from up to l192 active trunks. In interleaving, a first bit from a first character is assigned to the data field, followed by a first bit from a second character, etc. The resultant interleaving of bits from different characters eliminates a need to precalculate the number of bits of each character. Instead, bits are interleaved until the data field is full, which typically occurs during overload, or until all bits of a character from each active trunk have been transmitted, whichever event occurs first.
During overload, a character from one trunk, whether it be a speech sample or a data sample, may have truncated therefrom more or less bits than a character from another trunk. As a result, the fewer bit signal is more severly degraded than a signal having less bits truncated. To mitigate this problem, the prior art teaches a priority rotation arrangement wherein the order of assigning bits for interleaving in the data field is rotated relative to all trunks, whether active or inactive. For example, the trunk activity status for input trunks 25-32 may be signalled in the status field of frame 4 of the output multiframe, while the activity status for input trunks 161-168 may be signalled in frame 21. The prior art ordder of assigning bits to the data field of frame 4 would be: a bit from a character on trunk 25 is first assigned to the data field, if trunk 25 is active; else, a bit from a character on the first active trunk taking the trunks in the increasing trunk sequence 26 through 192, wrapping around to trunks 1 through 24. Similarly, in frame 21, the prior art teaches the assigning as starting with trunk 161, if active; else with the first active trunk in the increasing trunk sequence 162 through 192, wrapping around to trunks 1 through 160. If, for purposes of illustration, we assume trunks 25 and 161 as being the only active trunks, it is clear that the prior art assigning for frame 4 would start with the first bit of the character from trunk 25. However, for each of output frames 5 through 21, the prior art rotation arrangement would start the interleaving with the first bit of the character from trunk 161. In such an event and during overload, signals from trunk 161 are typically accorded less signal degradation during each of frames 5 through 21 than would be accorded signals from another trunk. Hence, the prior art suffers from a nonuniform signal degradation problem.
Accordingly, it is a road object of the present invention to mitigate signal degradation during overload in a digital communication system.